Bass into a Rat pedal (or similar distortion with a low pass filter) and the highs rolled off, distortion up so it catches the peaks. If you've got an amp, great, use it, but a decent amp sim is fine too - the free UAD SVT is great. Chuck it into a slow squashy LA2A type compressor for consistency, then out to a pultec style EQ with the bass boosted (and if it's sounding boomy, the alternative bass attenuated on the other EQ). You might also need to boost the mids to get the clunk good and prominent. For that sound I'd agree with others, new strings are a big part of it.
Bass into a Rat pedal (or similar distortion with a low pass filter) and the highs rolled off, distortion up so it catches the peaks. If you've got an amp, great, use it, but a decent amp sim is fine too - the free UAD SVT is great. Chuck it into a slow squashy LA2A type compressor for consistency, then out to a pultec style EQ with the bass boosted (and if it's sounding boomy, the alternative bass attenuated on the other EQ). You might also need to boost the mids to get the clunk good and prominent. For that sound I'd agree with others, new strings are a big part of it.